1. Field of the Invention
The present invention relates to a developing device for use in copiers, facsimile devices, printers, and the like, and more particularly to a development device using a two-component developer comprising a toner and a magnetic carrier, an image forming apparatus using the developing device, and a toner and a carrier for use therein.
2. Description of the Related Art
A developing device using a two-component developer comprising a toner and a magnetic carrier is known. In such a developing device, a developing roller, which is a developer support, supports on the surface thereof a developer comprising a magnetic carrier and a toner with a plurality of magnetic poles provided inside the developing roller and conveys the developer to a zone facing a photosensitive body. A conveying path for feeding the developer to the developing roller and a conveying path for stirring the developer are provided separately from each other, and the developer is circulated by conveying the developer in opposite directions in the two conveying paths.
However, in such conventional developing device, the conveying path for feeding the developer to the developing roller and a conveying path for recovering the developer that was fed to the developing roller and has passed the development region are combined together. The resultant problem is that toner concentration in the developer fed to the developing roller decreases toward the downstream side in the conveying direction of the conveying path for feeding the developer to the developing roller. The decrease in toner concentration in the developer fed to the developing roller results in decreased image density during development.
This problem can be resolved by providing a screw for feeding the developer to the developing roller and a screw for recovering the developer after development in different developer conveying paths, as in the developing devices disclosed in Japanese Patent Applications Laid-open No. H11-167260 (FIG. 1) and 2001-290369 (FIG. 2).
However, in the developing devices suggested in these publications, as will be described below with reference to the appended drawings, excess stress is applied to the developer and shortens the service life thereof. Yet another problem is that toner concentration in the developer on the developing roller becomes uneven and image density becomes non-uniform.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Applications Laid-open No. H11-084874, H11-133710, 2001-249545, 2003-263025, and 2003-263026.